The present invention relates generally to the rotor of a superconducting generator and, more particularly, to the radiant heat shield used therein.
A superconducting generator typically has a supercooled rotor which includes a superconducting field winding and a structure for supporting the winding. During operation, the rotor is supercooled to a cryogenic temperature by circulating a fluid refrigerant, such as helium, proximate its conductive components. During normal operation of the generator, the liquid helium within the rotor is transformed into gas by a relatively slow but constant boiling.
The function of a superconducting rotor's radiant heat shield is to intercept heat that is radiated from the rotor's ambient surroundings, which are typicaly at room temperature, so as to prevent this radiated heat from raising the temperature of the cryogenic cold zone within the rotor. Typically, a radiant heat shield consists of a tubular structure disposed radially outward from the superconducting rotor field coils. This tubular structure is provided with a plurality of coolant channels therein along with a means for the refrigerant to pass radially into the radiant heat shield, move axially through it the coolant channels exit, via another radial passageway, toward the internal portion of the superconducting rotor. U.S. Pat. No. 4,250,418 issued to Eckels on Feb. 10, 1981 and copending U.S. patent application Ser. No. 143,205 filed on Apr. 24, 1980, now U.S. Pat. No. 4,312,149 by the present inventor and assigned to the present assignee are incorporated by reference herein and disclose, inter alia, particular designs of radiant heat shields. One object of the present invention is to provide a radiant heat shield for use with a superconducting generator which is designed to be manufactured in a reliable and yet economical manner.
The size of the radiant heat shield, in a typical superconducting generator, could exceed 130 inches in length and 30 inches in diameter and, since the radiant heat shield must be cooled with a plurality of coolant passages located within its cylindrical walls, these design parameters essentially require that it be made of a multiple-shell construction. The metallurgical bonding of these two shells must provide for an effectively sealed coolant channel network and must not distort the radiant heat shield nor weaken its structural integrity.
A radiant heat shield made in accordance with the present invention comprises an inner cylindrical tube and an outer cylindrical tube associated in coaxial and concentric relation. The inner tube has a network of coolant channels formed in its outer cylindrical surface. This coolant channel network can comprise two circumferential grooves located a predetermined axial distance apart from one another along with a plurality of axially extending grooves, with each axial groove intersecting and connecting the two above-described circumferential grooves. In this particular configuration of the coolant channel network, each circumferential groove is also provided with one or more radial holes which intersect it and provide fluid communication between it and the internal portion of the inner tube.
An outer cylinder is disposed radially outward from the inner tube and is metallurgically bonded to the inner tube to form a unitary radiant heat shield structure. The outer tube encloses the radially outward portion of each of the above mentioned grooves and thereby provides an enclosed coolant channel network which, in turn, provides fluid communication between the radial hole or holes which intersect one of the circumferential grooves and the radial hole or holes which intersect the other circumferential grooves.
In order to provide a suitable metallurgical bond between the inner and outer tubes, the outer cylindrical surface of the inner tube is coated with a brazing material prior to compressing the inner and outer tubes together. This compressing can be accomplished by co-cold rolling the inner and outer tubes together. As an alternative to co-cold rolling the tubes together, a heat shrinking operation can be used to assemble the inner and outer tubes together with a diametrical interference fit. Furthermore, the brazing material can be applied to both the inside cylindrical surface of the outer tube and the outside cylindrical surface of the inner tube. After the inner and outer tubes are compressed to form an intimate contact therebetween, the inner and outer tubes are heated to a temperature that exceeds the melting temperature of the above described brazing compound. Typical brazing compounds that may be used in accordance with the present invention are ones that contain 85% silver and 15% manganese, 7% silver with 85% carbon and 8% tin or a compound which contains boron, silicon and nickel. These brazing compounds have melting temperatures of 1760.degree. F.-1780.degree. F., 1225.degree. F.-1805.degree. F. and 1900.degree. F., respectively. It should be understood that other suitable brazing compounds are included within the scope of the present invention.
Also, in order to provide a more reliable brazed joint between the inner and outer tubes, the inner tube may be nickel plated prior to the application of the brazing compound as described above. In accordance with the present invention, the nickel plating and brazing compound application are done in such a way as to avoid affecting the axially outboard portions of the cylindrical outer surface of the inner tube. The reason for this is to avoid contamination of this outer cylindrical surface of the inner tube in the regions of its axial termini. This is done to prevent any deleterious affect on a later welding operation which provides a seal weld between the inner and outer tubes at their axial termini.
Prior to the inner and outer tubes being cold worked or heat shrunk together, a removable substance may be disposed in the above mentioned channel network. This removable substance prevents any localized deformation of the outer tube into the channels during the compressing working operation. This substance can then be removed following the compressing procedure. Although many removable substances are available for this purpose, a meltable substance such as a low melting metal or sulphur, a combustible substance, such as polystyrene, or an etchable substance, such as carbon steel, may be used.
Following the compressing operation, the assembly can be heated to a temperature of approximately 1950.degree. F. for one hour. This raised temperature melts the brazing material that has been disposed between the inner and outer shells, performs a solution annealing of the weld metal used in the above mentioned seal weld and provides a solution annealing of the inner and outer shells. The assembly can then be heat treated by raising its temperature to approximately 1400.degree. F. for five hours followed by 1200.degree. F. for eight hours in order to properly age the components of the radiant heat shield. Finally, the assembly may be machined to final dimensional tolerances.
It should be apparent that the present invention provides a method of construction of a radiant heat shield which has good contact between the adjacent surfaces of the inner and outer tubes and prevents leakage of coolant either between adjacent axial grooves or between the coolant channel network and the surrounding environment which is typically a vacuum. It should further be apparent that the present invention provides a radiant heat shield, for use with superconducting rotors, that is manufacturable in a manner which results in a structure of high mechanical integrity and which also provides a reliable fluid containment for the superconducting rotor's refrigerant.